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In this section the key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" are to be interpreted as described in [RFC2119].
This specification defines three levels of conformance, in order of completeness:
includes the set of formatting objects and properties needed to support a minimum level of pagination or aural rendering.
includes everything else, except for shorthands. It is intended for applications whose goal is to provide sophisticated pagination.
Conformance to this specification is expressed in terms of conformance to any of the above levels.
An application that claims conformance to a given level of this specification must implement all the formatting objects and properties that apply to it for a given medium.
[B Formatting Object Summary] specifies which formatting objects belong to each of the above levels, and for what medium.
[C Property Summary] specifies which properties belong to each of the above levels.
The minimum level of conformance is Basic. A minimally conformant implementation must process as specified all the formatting objects and properties defined for the Basic level of the implementation's target medium.
Implementations may choose to process formatting objects from levels or target media other than the one to which they conform. In order to ensure interoperability, this specification defines a fallback for each formatting object in the Extended and Complete levels.
An implementation must not claim conformance to a given level if any of the formatting objects at that level is implemented solely as the fallback specified here for that level. Correct processing of fallbacks does not constitute conformance.
Conforming implementations must support at least one of the "writing-mode" values defined in this Recommendation. Although writing-mode is defined as a Basic property with an initial value of "lr-tb", it is not the intention of this specification to impose this particular, or any other, writing mode value on conformant applications. If an implementation does not support a writing-mode used in a stylesheet, either explicitly or by relying on the initial value, it should display either a 'missing character' glyph message or display some indication that the content cannot be correctly rendered.